Applicant is one of the inventors in copending U.S. application, Ser. No. 09/863,571, now U.S. Pat. No. 6,420,782 D2 for xe2x80x9cMethod for Sample Separation and Lift-Out,xe2x80x9d filed May 23, 2001.
The present invention relates to a method for separating a sample, and a method for preparing the separated sample for analysis, in cases where analysis is desired; and particularly relates to a method for separating a minute sample region from a substrate such as a semiconductor wafer.
This application describes embodiments in which a sample is cut out of a semiconductor wafer or other object by use of a focused ion beam (FIB) for further processing, modification, or analysis, and analyzed, if desired, through the use of a transmission electron microscope (TEM), scanning electron microscope (SEM), or by other means.
Certain inspection methods of samples from integrated circuit wafers and other materials require the fabrication of an electron-transparent ( less than 50 nm thickness) region on the sample that contains the place of interest for observation. The traditional means for preparing a sample for TEM inspection involves isolating the region of interest by means of a diamond wafering saw, precision wafer cleaving apparatus, or mechanical grinding. With this method, it is difficult to set the place of observation and direction of the sample desirably and precisely. It is necessary to carry out a step in which a region having a length of several mm, a width of 100-500 xcexcm, and the thickness of the semiconductor wafer, and including a portion to be analyzed, is mechanically separated from the chip of an integrated circuit or semiconductor wafer. In the case of a semiconductor wafer, this requires dividing the wafer and precludes the further use of this wafer for production. Additional steps involving mechanical grinding, polishing and low energy ion milling ( less than 3 keV Argon), are required to thin the excised portion in certain areas to the thickness required for TEM inspection. This procedure is acceptable for the inspection of repetitive structures, but is challenging and time consuming for site-specific inspections in which a specific micron-sized target region must be exposed for TEM inspection.
The use of the FIB offers advantages over conventional mechanical TEM sample preparation due to its ability to inspect the integrated circuit wafer and use the ion beam to thin the sample to the thickness required for TEM inspection. In an established method for TEM sample preparation in a FIB instrument, a chip, or xe2x80x9cribbon,xe2x80x9d having a length of several mm, a width of 100 xcexcm to several mm, and the thickness of the semiconductor wafer, and containing the region of interest for observation, is cut out from a semiconductor integrated circuit wafer by use of a diamond wafering saw, precision wafer cleaving apparatus, or mechanical grinding, or in some combination of these techniques. In the case of a semiconductor wafer, this requires dividing the wafer and precludes the further use of this wafer for production. The ribbon is then mounted on a modified TEM grid. The portion of the ribbon containing the region of interest is formed into an electron-transparent sample by focused ion beam milling in the FIB. The thin film region of the sample can then be inspected in the TEM by irradiation with an electron beam.
A method for performing the entire TEM sample preparation within the FIB known as xe2x80x9cin-situ lift-outxe2x80x9d relies on a mechanical probe within the sample vacuum chamber of the charged-particle instrument to extract the excised sample containing the region of interest once it has been released from the wafer by ion milling in the FIB. In-situ lift-out typically uses the gas-assisted material deposition capability of the FIB to connect the excised sample containing the region of interest to the tip of the mechanical probe. The excised sample containing the region of interest can then be attached to a modified TEM grid by means of the gas-assisted material deposition capability of the FIB, and separated from the mechanical probe by ion milling in the FIB. The region of interest can then be thinned to an electron-transparent thickness using ion milling in the FIB. In a previously practiced method for in-situ lift-out (U.S. Pat. No. 5,270,552 to Ohnishi, et al.), the tip of the mechanical probe is connected to the smaller sample before the sample is completely released from the wafer. This practice has disadvantages overcome in the present invention.
Due to re-deposition of material during charged-particle milling, it is often difficult to determine if the cuts being made into the material are complete. There is a danger the operator using the conventional method will damage the sample by prematurely attempting lift-out. There is also a possibility that more time than necessary will be spent on the charged-particle milling to ensure that material re-deposited during the milling process has been completely removed.
Further, if the mechanical probe is connected by material deposition to the sample containing the region of interest, and the charged-particle milling is performed from two different angles relative to the surface of the wafer, re-deposition during the second milling operation into the cut from the first milling operation may require tilting the sample back to the charged-particle incident angle used for the first milling operation to re-open the initial cuts. This would require disconnecting the mechanical probe to re-do the first milling operation, re-orienting the sample to the final milling angle, and then re-connecting the mechanical probe to the smaller sample for lift-out, possibly damaging the sample. Even more important is the considerable increase in time to complete the inspection operation if the probe must be disconnected and re-connected.
Even though the mechanical performance of the in-situ mechanical probe can be optimized for typical operating conditions of the charged-particle instrument, unexpected transient mechanical events may produce a force that will result in relative displacement between the probe and sample, or a stress on the connection between the probe and sample or on the sample itself. Such stress can break the attachment between the probe and sample, and possibly damage the sample. Again, re-connection of the probe causes unacceptable delay in the inspection operation.
There is a need for an in-situ lift out method that avoids the dangers of premature lift-out; that allows re-orientation of the sample for additional milling without modifying the sample; and, that minimizes the time during which a transient mechanical event can place damaging stress on the probe and sample or cause unacceptable delays. The present invention solves all of these problems inherent in the prior art.
The invention is preferably embodied in a method for sample separation and lift-out comprising the following steps:
First, a wafer, usually but not necessarily a semiconductor device, is positioned inside a FIB instrument. The wafer will have an area of interest, or target. The ion beam is positioned at substantially normal incidence to the plane of the wafer. In an alternative embodiment, the first position of the ion beam will be at some acute angle to the plane of the wafer.
The ion beam is used to cut a substantially U-shaped first cut into the wafer; the U-shaped first cut at least partially surrounds the target. Next, the wafer is re-positioned with respect to the ion beam. Then the ion beam is preferably positioned at an angle in the range of 45 to 60 degrees to the plane of the wafer.
A second cut is made in the wafer with the ion beam, undercutting the target, so that a sample is completely released from the wafer. Next, a probe is fixed to the released sample, preferably with ion-beam deposition, and the sample and the wafer can then be separated.
The order of the cuts may be reversed. In this case, the ion beam is positioned at an angle less than 90 degrees to the plane of the wafer. Then the ion beam is used to make a first cut in the wafer, undercutting the target. Next, the wafer is repositioned with respect to the ion beam by turning the wafer approximately 180 degrees, and the ion beam is re-positioned to substantially normal incidence to the plane of the wafer. The ion beam then cuts a U-shaped second cut into the wafer; the U-shaped second cut at least partially surrounding the target and intersecting the first cut, so that the sample is completely released from the wafer. A probe is attached to the sample as before.